The demand for high-performance polypropylene homopolymer and copolymer products has resulted in advanced manufacturing processes that have unique features. Gas-phase polymerization processes have been recognized as the most economical for the manufacture of polypropylene homopolymers, random copolymers, and impact copolymers. In such processes, the polymerization reactor contains a mixture of a solid bed of polymer particles and a gas phase that comprises propylene monomer, comonomer and hydrogen. Unlike slurry-phase polymerization processes, gas-phase processes do not require solid-liquid separation or product-catalyst separation. This feature makes the gas-phase processes easier to operate and more economical.
Operational issues may occur in gas-phase polymerization reactors if the process is not properly controlled. For example, polymer agglomerates or even polymer chunks may form and/or vessel surfaces may foul. These issues are typically caused by inadequate heat removal and/or strong electrostatic adhesion. When the heat of reaction is not removed rapidly, the polymer particle heats up to a surface temperature higher than the polymer softening temperature or even higher than the melting temperature of the polymer. Such operational problems can force the shutdown of the reactor for clean up, resulting in severe financial penalties.
In reactors with relatively-high bed bulk densities there usually are more active catalyst sites (sites where the polymerization reaction occurs) in the unit bed volume and more reaction heat generated in the unit bed volume. Moreover, the relatively-high bed bulk density is often associated with, or caused by, a relatively-low gas velocity, and hence a relatively-low heat removal capability by the gas phase. This further increases the possibility of particle agglomeration. The term “bed bulk density” refers to the weight of solids in the unit volume of the gas-solid bed in the reactor. This term is often equivalent to “fluidized bulk density (also known as “FBD”)” when the gas-phase reactor is a fluidized bed reactor. For the purposes of this invention, when extending this concept to the non-fluidization gas-phase reactor in this invention, such as a mechanically stirred vertical gas-solid reactor with gas velocity smaller than the minimum fluidization velocity, both terminologies can still be used to mean the weight of solid per unit volume of the gas-solid system.
Previous attempts to solve the above-mentioned agglomeration problems include, for example, mechanical agitators added into both the vertical gas-phase reactors (e.g., U.S. Pat. No. 3,639,377) and horizontal gas-phase reactors (e.g., U.S. Pat. No. 4,101,289). However, the mechanical agitation does not always solve the agglomeration problem, and the agitator itself provides additional surface for fouling. The “chunk” formation in gas-phase reactors with agitator is well reported, such as CN 101241023, and CISEC Journal, Vol. 60, No. 3, pp. 585-592. In the latter document, it is stated that “in horizontal stirred-bed polypropylene reactor, chunking is a severe threat to the long-term stable operation and product quality of the reactor.” The authors of that paper even further pointed to the polypropylene reactors of Innovene and Chisso processes, which are considered as reactors with relatively high bed bulk density, compared with other gas-phase fluidized-bed reactor operated under relatively high velocities, such as the reactors of The Dow Chemical Company's UNIPOL™ polypropylene process.
Therefore, the particle agglomeration in the gas-phase polymerization reactors with high bed bulk density could be a particular operational concern, and there is a need to develop a solution to the problem. The present invention provides an easy-to-apply, relatively low cost and low operational complexity, solution of this problem, without the need for any significant change of process equipment and operating conditions. The present invention is an improvement for gas-phase polymerization processes having a polymer bed with a bulk density greater than 128 kg/m3. The improvement involves the use of a mixed external electron donor feed, wherein the mixed electron donor system comprises at least a first external electron donor and a second external electron donor, and wherein the first external electron donor is a carboxylate compound.